The invention relates to synchronous electrical machines and particularly but not exclusively to synchronous alternators for converting mechanical power applied on a rotating input shaft of the alternator into electrical alternating currents (a.c.).
The use of superconductors for at least some windings of a rotating electrical machine has already been proposed, the great attraction of superconductors being that very heavy currents can flow through them with very low losses and, correspondingly, the conductor size required for a particular current flow is very small indeed. However, a superconducting winding be maintained at a very low temperature. The direct coupling between the rotating field winding and the driving plant in known systems forms a heat leakage path and a very high power cooling or refrigerating facility should be provided.
Electrical machines having a rotating superconductive field winding are also known wherein a shaft having a short-circuited winding or squirrel cage is disposed between the field winding and a stationary stator winding, the shaft receiving the mechanical input in the case of an alternator and providing a mechanical output in the case of a motor. Machines of that type are disclosed in French Patent Specification No. EN 72 38 967 of the assignee of the present invention corresponding to U.S. patent application No. 412,127 as well as in U.S. Pat. No. 3,742,265 (Smith). However, such machines are inherently asynchronous (i.e. the rotating magnetic field of the shaft slips relatively to the field produced by the field winding) and for many uses and particularly electricity production synchronous operation is preferable.